Trees and pavement   Robert Leverett
  Nov 15, 2005 10:10 PST 


In urban areas I've often seen large trees surrounded by pavement that
goes right up to within a couple of feet of the trunk. The pavement
might extend in all directions for quite a distance and channel runoff
into the drainage system. I've often wondered how a large tree in such
conditions is able to maintain itself. I assume even though there is a
pavement covering that the tree has some source of underground water
available to it. Lots of questions here. Would you mind casting some
light on this subject? Thanks.

Re: Trees and pavement
  Nov 15, 2005 12:18 PST 

You hit it on the head. Trees seek out water. It could have it's roots in a sewer line, a water line, a natural spring. Depending on the tree, the roots can extend over 2.5 times the height of the tree. Here on our property there is a European Beech that is very close to the state champion status. It is 62" dbh and 80'+ tall. It is planted within 40' of a building and surrounded by sidewalk. I often wondered how it did so well. During a construction project, we discovered a huge brick chamber, that collects downspout water, within the root zone of the tree. We do not yet know the length or width, but the water was eleven feet deep!! It was also full during the drought. This tree is about 160 yrs old, and this constant water source has to be a factor to it's survivability. Sometimes a large tree is paved up to it's roots, and it starts to die. Often a large tree can live for many years after the critical blow has been delivered. Example: We removed a 187 year old Black oak (ring count) in 2004. The root zone was cut through during the installation of public sewer in the 60's, and in my opinion was the cause of death. Another example: We removed a 178 year old White oak (ring count) that had been buried 2' in 1961. We noticed the tree was under heavy attack by Gypsy moth caterpillars, we excavated the soil away, and the tree died the next year. My point is those big trees you see with pavement up to the trunk may be dying as we speak, just not dead yet.
   If a tree is newly planted in these conditions, it stands a better chance at living. I think the change to the trees environment is too shocking. As mentioned some trees fair better in certain circumstances.

RE: Trees and pavement   Will Blozan
  Nov 15, 2005 12:51 PST 

Bob, Scott...

I have heard conflicting ideas on pavement. Some say pavement is a barrier
to evaporation and as such keeps the soil beneath constantly moist, i.e. it
can't dry out. However, soil oxygen is in short supply. Floodplain species
such as American elm and American sycamore (and blackgum as well) seem to be
able to deal with paving OK (at least relative to other species) since they
can tolerate anaerobic soil conditions for extended periods of time.

I consulted on a large elm (3' dbh) here in Black Mountain that had been
covered under the entire drip line with hot asphalt mix. They even ran the
mix up the root (trunk) flares. I pronounced the tree doomed. Anyway, the
tree is currently perfect and that was in 1987. I suspect the roots were
able to exploit areas beyond the pavement; in fact they were likely already

Trees are cool!

Will B
RE: Trees and pavement
  Nov 15, 2005 13:43 PST 

I wonder if that Elm has a water source underground, or it may not be dead yet. It can take along time, although paving is different than cutting through a root system close to the trunk. Those big oaks took 40 years to die.

roots, mulch, paving & soil compaction
  Nov 16, 2005 12:51 PST 

Trees installed into vaults in paved areas tend to explore what is available
to them, but, in truth, they really become "temporary" trees. Their root
systems may actually fill the available space, and compress the soil to a point
where the soil will not hold moisture. This can also be the case with trees grown
in large boxes or tubs.

In hindsight, I regret that I have not written down the upteen "old wives
tales" that encompass tree roots, as they would have made a very interesting
book! ... tap roots grow 40 feet deep... roots seek water... roots are only under
the canopy... you cannot plant trees within 50 feet of a foundation... roots
grow straight down... you cannot transplant Post Oaks.... Folks in rural Texas
tend to anthropromorphize trees and their components.

I once saw Post Oaks dying 300 feet upslope from a pasture where Velpar
herbicide was applied to kill Mesquite stumps. We may never know, or have the
ability to determine, how far roots range from a given tree.

I have also seen Live Oaks killed where herbicide residue was washed , by
rainfall, across a parking lot from a railroad ROW... the tree roots were
apparently under the concrete at a range of 150 ft. +, from the trees, and the
diluted chemicals seeped into the expansion joints.

It appears that, the condensate resulting at the interface of the subsoil and
bottom of the concrete, is adequate to support some tree species. Also, air
apparently does infiltrate the expansion joints and perhaps some rainfall.

The never-ending disagreement between structural engineers and arborists over
tree roots damaging house foundations. I have seen massive root invasion and
foundation lifting due to roots (Live Oak) following the original sewer/water
trench under an engineered slab foundation. These trenches are never compacted
and therefore, appear to be reasonably aerated and moist. The small roots
infiltrate the boundry between the slab and pad soil and when they expand
diametrically, they act like a jack.... amazing stuff!

Tree roots are THE most important part of a tree and the part least

G. Sandy Rose, RCA
Registered Consulting Arborist
Shade Masters, Inc.
Arlington, Texas
(Pre-) Trees and pavement   Robert Leverett
  June 03, 2003 

There's always a reason. Thriving in paved-over areas due to the species adaptation to oxygen-deprived environments made immediate sense - once you said it. I'm now thinking of the cottonwood corridors I regularly pass that have developed adjacent to paved areas. Of course! 


Lee Frelich wrote:  (06-03-03)

Cottonwood only grows into a big tree (15' cbh) on deep soils, and also it is extremely intolerant of shade and cannot compete (especially at the seedling stage) with other species that are more shade-tolerant once you get far from the river. Cottonwood can function as an early successional species after a major disturbance (tornado, flood, fire, clearcutting), regardless of distance from the river. But it cannot grow in its own shade,
and it only dominates for long time periods in areas where there is frequent flooding, since it can better survive the period of root saturation and lack of oxygen than other species, and it is even a little better at that than silver maple. The seedlings can grow to heights of 10 or 20 feet in a year or two, thus getting above the flood levels of all but the greatest floods (we have 40 foot floods in the Midwest, but only every 10 or 20 years, a typical spring flood is only 10-15 feet). Any seedling that is totally submerged for more than a few hours will die.

RE: Trees and pavement   Steve H.
  Jun 22, 2006 19:20 PDT 

Will, Bob, ...

Here in Framingham, MA where I live there is a beautiful American Elm
which is apparently in perfect health and is probably fairly old though
it is not a really "big" tree. It is completely surrounded by pavement
up to the trunk/roots and interrupts a sidewalk (I suspect it predates
the sidewalk). On one side of the sidewalk is a busy road and on the
other is a large parking lot which services a mini-mall which looks like
it was built in the early 1970's. Across the street is another sidewalk
and then after ten feet of grass is a nursing home with a sizable
parking lot.

The tree is in a low lying area of town and there is a large pond on the
other side of the mini-mall so I think it is getting plenty of water.
What amazes me is that the soil is probably poorly oxygenated and there
has been no chance of soil nutrient replenshiment for at least 30 years.
To make things tougher on the tree the road that it abuts is torn up
every ten or fifteen years to be repaved as it is one of the main roads
leading into the center of town; most recently last summer.

Somehow it survives nicely and apparently is immune to the ravages of
DED which wiped out almost every other elm in town many years ago.

Steve H.
RE: Trees and pavement   Robert Leverett
  Jun 23, 2006 05:02 PDT 


   On occasion I see a tree growing and thriving where no
self-respecting tree should be. I am alway amazed and puzzled since the
model that I carry in my head says that both water and air must be in
the soil around a tree's roots. Maybe Will, Lee Scott, etc. can shed
some light on this.
RE: Trees and pavement   Darian Copiz
  Jun 23, 2006 07:11 PDT 


It's one reason elms were planted so extensively - because they are
tough. A lot of the floodplain species have been used as street trees
because they can handle compacted soil, alkaline soil, and some other
adverse conditions better than others. Having sediment layered over
their roots and being banged up by floating debris is similar to some
urban situations. I don't quite understand why many of them are drought
resistant though or what biological mechanism gives them an advantage in
these tough situations.

RE: Trees and pavement   Steve Hewlett
  Jun 23, 2006 07:32 PDT 


I found the "trees and pavement" original messages through the archive
here: It
is the second topic listed under the heading "Tree Care".

I moved to Framingham a little over four years ago and the lonely elm
growing out of the sidewalk at the mini-mall caught my eye right away.
It is an absolutely gorgeous specimen living under really adverse
conditions. If it seeds this fall I will try and collect a few seeds. I
suspect that tree is quite old even though it's circumference is
proabably only about 7-8 feet or so with a height of about 70 feet. It
is in an old part of town and if it predates the sidewalk it grows out
of then it is probably at least a hundred years old or more. Framingham
was settled in the mid 1600's. It seems to me that trees that grow under
adverse conditions are not going to reach optimum or even average size
for the species, sort of an unintentional bonsai process.

RE: Trees and pavement
  Jun 23, 2006 08:09 PDT 

Part of the reason that certain trees can tolerate compacted soils, as well as very wet conditions, is that their roots don't require much oxygen. Both conditions are anerobic. Sycamore, Elm, Pin oak, red maple, and others are tolerant of wet and compacted environments.


I like the analogy of sediments covering the roots to asphalt and sidewalks covering the roots, and debris bumping into the trunks like cars do. Very interesting.

RE: Trees and pavement   Lee E. Frelich
  Jun 23, 2006 09:53 PDT 

Bob, Steve:

Elms don't need a lot of oxygen, since they are floodplain species. That
is why they do so well in cities.